Internal combustions engines combust an air and fuel (A/F) mixture within cylinders to produce drive torque. More specifically, the combustion events reciprocally drive pistons that drive a crankshaft to provide torque output from the engine. The A/F mixture is ignited or sparked at a desired crank angle. In some instances, however, the A/F mixture ahead of the flame-front auto-ignites within the cylinder resulting in undesired engine knock.
Accordingly, engine knock control systems have been developed to detect and to mitigate engine knock. One such engine knock control system is disclosed in U.S. Pat. No. 5,560,337, entitled Knock Control Using Fuzzy Logic, and issued on Oct. 1, 1996. Such traditional systems detect the propensity for a particular cylinder to auto-ignite and retard the cylinder spark timing to avoid engine knock. Although engine knock is avoided, exhaust gas temperatures increase as a result of the retarded spark timing.
Some internal combustion engines include a turbocharger, which increases the charge air density ingested by the engine. The turbocharger is driven by the exhaust gas, whereby the heat energy of the exhaust gas is transformed into mechanical energy to compress the air entering the engine. In turbocharged engines, engine knock will occur at high loads, especially when a low octane (e.g., 85 octane) fuel is used. Consequently, a persistent knock condition can occur.
Traditional engine knock control systems do not sufficiently mitigate engine knock when applied in a boosted internal combustion engine. More specifically, traditional engine knock control systems retard spark in order to mitigate engine knock. However, spark retard results in higher exhaust temperatures, which in turn result in increased boost of the turbocharger (i.e., higher temperatures means higher heat energy, which results in increased turbocharger boost). Consequently, engine knock actually increases as a result of spark retard. Accordingly, the spark retard is continuously increased until the spark retard authority is fully consumed.